# OpenGL 2D - Rotation distorts sprite

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Hi,

I've been trying to create my own Sprite class in C++/OpenGL (and GLM/SFML/SOIL). I sorted the problem of rotating the sprite around its center but the problem I am now facing is the distortion of the sprite when rotated. Here are a few screenshots:

Here is the code for the matrix tranformations:

	glm::mat4 model, projection, mvp;

/* Model matrix */
/* Translating the sprite to its position and scaling it */
model = glm::translate(model, glm::vec3(displayPos.x / SCREEN_WIDTH * 2, -displayPos.y / SCREEN_HEIGHT * 2, 0.0f));
model = glm::scale(model, glm::vec3(m_scale.x, m_scale.y, 0.0f));

/* Rotating the sprite (we translate it to its origin, rotate it and translate it back to where it was) */
glm::vec2 origin;
origin.x = ((SCREEN_WIDTH / 2 - m_size.x / 2) / SCREEN_WIDTH * 2);
origin.y = -((SCREEN_HEIGHT / 2 - m_size.y / 2) / SCREEN_HEIGHT * 2);

model = glm::translate(model, glm::vec3(-origin.x, -origin.y, 0.0f));
model = glm::rotate(model, m_rotation, glm::vec3(0.0f, 0.0f, 1.0f));
model = glm::translate(model, glm::vec3(origin.x, origin.y, 0.0f));

/* Projection matrix */
projection = glm::ortho(0.0f, (float)SCREEN_WIDTH, (float)SCREEN_HEIGHT, 0.0f, 0.0f, 1.0f);

mvp = model; //I do not multiply the projection mat because it causes the sprite to disappear


I understand that I should normally be using glm::ortho somewhere, though when I use it, it just causes the sprite to disappear.
Also note that when I put the dimensions of the screen to a square (like 800x800 instead of 800x600), it works fine. I tried using glViewport(0, 0, SCREEN_WIDTH, SCREEN_HEIGHT) but it doesn't change anything.
The image is a 320x320 square.

Thanks.

Edited by Dexario

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I believe it is because you are using a non-uniform scale in the model-view matrix.

model = glm::scale(model, glm::vec3(m_scale.x, m_scale.y, 0.0f));

//I do not multiply the projection mat because it causes the sprite to disappear

This is where the viewport correction scaling is supposed to happen.  This is where you should be looking to fix.

Don't try to viewport correct your model in world space.

Edited by Vulkanizer

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No, I commented out this line, nothing changed and in addition to that, m_scale.x = m_scale.y = 1.0f, which doesn't change anything.

Also, if I scale up/down and rotate, it behaves exactly like the case I presented but with a bigger or smaller image, so I don't think that is the source or the problem.

Edited by Dexario

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Without using projection matrix it's like you've send the data in clip coordinates, and with the w set to 1 which is the NDC coordinates, in this space the width of the screen is equal to the height of the screen, since the top right point on the screen is (1,1) and the down left is (-1,-1), and to display the image then these coordinates are scaled to fit the view port, so if you see a square on the screen in this coordinates that square's coordinates are actually not one for a square, so rotating it will cause the distortion.

What the projection matrix does is to scale the view space coordinates before transforming them to clip coordinates, this solves the problem, though you aren't using a projection matrix.

You can use a scale matrix instead of the projection matrix, scaling the x by h/w or y by w/h (w being the width of the screen and h being its height).

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What the projection matrix does is to scale the view space coordinates before transforming them to clip coordinates, this solves the problem, though you aren't using a projection matrix.

I'd like to use a projection matrix, it's just that it doesn't seem to work:

	/* Projection matrix */
projection = glm::ortho(0.0f, (float)SCREEN_WIDTH, (float)SCREEN_HEIGHT, 0.0f);

mvp = projection * model;


I've seen on the internet that this should do the trick, however, it just displays a white pixel at the top left of the window for me.

Edited by Dexario

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sorry, my mistake.

Edited by Vulkanizer

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What the projection matrix does is to scale the view space coordinates before transforming them to clip coordinates, this solves the problem, though you aren't using a projection matrix.

I'd like to use a projection matrix, it's just that it doesn't seem to work:

	/* Projection matrix */
projection = glm::ortho(0.0f, (float)SCREEN_WIDTH, (float)SCREEN_HEIGHT, 0.0f);

mvp = projection * model;


I've seen on the internet that this should do the trick, however, it just displays a white pixel at the top left of the window for me.

projection = glm::ortho(0.0f, (float)SCREEN_WIDTH, (float)SCREEN_HEIGHT, 0.0f);


That will flip your image, which is you have face culling will result in the quad not being drawn.

Use:

projection = glm::ortho(0.0f, (float)SCREEN_WIDTH, 0.0f, (float)SCREEN_HEIGHT);


though i'd myself rather:

projection = glm::ortho(-(float)SCREEN_WIDTH/2.0f, (float)SCREEN_WIDTH/2.0f, -(float)SCREEN_HEIGHT/2.0f, (float)SCREEN_HEIGHT/2.0f);


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Using

projection = glm::ortho(0.0f, (float)SCREEN_WIDTH, 0.0f, (float)SCREEN_HEIGHT);


Doesn't display anything, though using

projection = glm::ortho(-(float)SCREEN_WIDTH/2.0f, (float)SCREEN_WIDTH/2.0f, -(float)SCREEN_HEIGHT/2.0f, (float)SCREEN_HEIGHT/2.0f);


Displays the image at the right position (or it seems to be) yet it is absolutely miniscule (about 2x2 pixels)

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The reason nothing is drawn with the first projection matrix might be the position of the quad, since it doesn't take the 0,0 of the scene as origin but the ((float)SCREEN_WIDTH/2.0f, (float)SCREEN_HEIGHT/2.0f), the reason things are miniscaled is that projection matrix scales the scene x by the h/w

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Basically, the square is at the top left of the screen in the OpenGL coordinate system.

Is that right or was I supposed to center the quad (put its center at 0;0 in the OpenGL coordinate system) ?

Edited by Dexario

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